Method and device for directional down-hole drilling

ABSTRACT

The invention concerns a method and a device for directional drilling of a hole, in particular when drilling curved holes for mining, drift mining or similar, whereby a drill support is used in the form of a flexible continuous drill tube with a proximal end and a distal end, that at the end of the drill support distal end is connected a down hole assembly comprising a drill bit for forming a drill string, which is fed into the gradually forming hole. To avoid and minimise the occurrence of “stick slip” during directional drilling, according to the method, the drill support while drilling is taking place is held fixed at its proximal end against rotation outside the borehole and the rotating motion for indexing the drill bit around an axis extending in the direction of drilling is performed in the area between the drill support&#39;s distal end and the drill bit while drilling is in progress.

The present invention concerns a method for directional down-holedrilling as described in the preamble to claim 1. The invention alsoconcerns a device for carrying out the method.

For directional drilling in rock, drift mining or blast hole and welldrilling a drill support in the form of coil tubing is used, which meansthat instead of a series of conventional rigid drill tubes a flexiblecontinuous drill tube with one proximal and one distal end is used. Theflexible drill support is unreeled from a spool and fed successivelyinto the borehole. To form a drill string, a continuous bottom holeassembly (BHA), which includes a drill machine, is fastened at thedistal end of the drill support. A percussion unit in the form of afluid-pressure activated hammer or similar reciprocal mass is arrangedon the drill machine to apply a drill bit fastened to the drill machineaxially against the opposing rock surface. The bottom hole assembly(BHA) usually also comprises some kind of positioning device, so-calledmeasurement while drilling (MWD). A driving fluid intended for themotors included in the BHA is led via a passage extended through thedrill support. The rock cuttings are transported away from the workedarea and out of the borehole with the used driving fluid.

In order switch the contact points of the drill bit studs and to makethem work on uncut rock, the drill machine must be indexed or graduallyrotated around its axis between each blow of the drill bit. This isnormally achieved by rotating or twisting the flexible drill supportwith some kind of rotating arrangement situated outside the borehole.

For directional control of the drill bit inside the borehole, there is asteerable motor arranged in the BHA that conveys motion through auniversal joint or other suitably flexible part so that the drill bitcan take on different angles relative to the drill support. The said BHAnormally constitutes a fluid activated motor in the form of a so-calledmudmotor that, like the other motors in the BHA, is supplied with adriving fluid via a passage extending through the drill support.

When drilling deep holes, so-called “stick slip” can arise due to therotation of the drill support, which at its proximal end is continuousor uniform, becoming irregular at its distal end so that the drillsupport acts like a torsion spring instead of indexing the drill machineevenly between blows from the percussion unit. This means that the drillmachine will be standing still for several blows and accumulating torquebefore running away uncontrollably while performing a very rapidrotation. This “stick-slip” effect reduces the penetration rate andincreases bit wear.

One considerable difference between conventional linear drilling withdrill support comprising a series of joinable drill tubes and a flexibledrill tube of coil string type is that the use of a flexible drillsupport allows only a relatively small application force on the drillbit, considerably reducing the efficiency of the drilling technique. Inparticular with regard to directional drilling of curved holes, thislimitation is tangible because a large part of the application force islost in each part of the drill tube that bends. This problem is duepartly to geometric conditions but also partly to the large staticforces of friction that arise at each deflected part of the drillsupport. In this part, it should be understood that the aforesaidproblems become greater as the curve radius becomes smaller.

Recently, it has become increasingly common to use directional drilling,which puts higher demands on equipment efficiency and especially whendrilling series of non-linear holes, i.e. curved or crooked holes. Oneexample of this is in cases where it is necessary to avoid drilling innon-ore bearing rock but to efficiently control the drilling directiondirectly towards the ore-bearing bodies in the rock.

One objective of the present invention is to minimise the occurrence of“stick slip” during directional drilling, in particular when drillingcurved holes with small radii because the forces of friction that herebyarise between the flexible drill support and the rock or wall of thehole become very large when rotating the drill support for indexing ofthe drill machine.

These objects of the invention can be achieved with the device accordingto the present invention having been given the distinctive features andcharacteristics specified in the claims.

The following is a more detailed description of this invention withreferences to the only attached drawing on which is shown a schematic ofan embodiment of the present invention.

Referring to FIG. 1 is shown an example of the equipment that accordingto the invention is used for directional drilling in the transition froma vertical section 1 to a horizontal section 2 of a borehole 3 in rock.At the distal end of a tubular flexible drill support 4 of so-called“coil string” type is arranged a bottom hole assembly (BHA) generallydesignated 5. 6 designates a drum on which the flexible drill support 4is wound and 7 a means of feeding the drill support down into thegradually formed borehole.

In order to form something that below is called a drill string, thebottom hole assembly 5 comprises, viewed from the distal end of thedrill support, a percussion tool 8, a positioning device 9 or aso-called “measurement while drilling” (MWD), a control device 10 in theform of a motor for inclining or setting the position of the assembly,an indexing device 11 and a drill machine 12 with drill bit 13. Theindexing device 11 is intended to gradually rotate the drill machinearound the main axis of the drill bit extending in the drillingdirection and is primarily of a suitable conventional design like otherunits included in the assembly such as the percussion tool, MWDequipment and the drill machine.

From a purely design point of view, the indexing device 11 can bedesigned in a number of different ways and in the embodiment describedherein comprises a rotatable fluid motor of mud type and a motor-drivenvalve arrangement used to turn the whole drill machine through a certainangle (index) between each blow. In operation, a drive fluid comprisinga mixture of water or a suspension of bentonite clay in water flowsthrough the motor, which in turn drives the valve arrangement. In aknown manner, the valve arrangement is so designed and balanced that avariation of the drive fluid passing through the motor will result in achange of the flow balance through the valve arrangement, which in turnis so connected to a motor or similar actuator included in the indexingdevice that the drill machine 12 is turned through a certain angle underthe influence of the motor.

In a similar manner, the other units included in the bottom holeassembly is equipped with motors and valve arrangements designed tocontrol and regulate through varying the flow of drive fluid passingthrough the motor.

In operation, the percussion tool 8 generates an axial reciprocatingmovement that is conveyed to the drill machine so that the drill bit 13arranged in the same attains a drilling action against the opposing rocksurface. The drill bit 13 is gradually fed into the hole via a feeddevice 7 while it is applied with a certain force against the opposingrock surface. The feed device 7 holds the proximal end of the drillsupport torsionally fast against any torsional forces occurring in theborehole 3. Normally, the drill bit 13 is driven forward in this way ina linear direction inside the borehole. In a conventional manner, adirectional control of the drill bit 13 is performed through the actionof the control device 10 with motor and the adjustable joint connectedto it, whereby the direction can be checked with the measurement whiledrilling (MWD) equipment.

To make the axially reciprocating drill bit 13 studs continually act onunworked rock, the drill bit must be indexed or rotated in suitablesteps around the drill axis in conjunction with each blow. Using knowntechnology, this can be achieved with a means of rotation situatedoutside the hole.

According to principles of the present invention however, indexing isachieved in close conjunction with the drill machine by means of anindexing device 11 that is driven by fluid passing through a motorincluded in the indexing device.

The indexing device 11 is synchronised with the percussion tool in sucha way that the indexing and rotation of the drill machine 12 takes placein conjunction with the drill bit 13 moving away from the rock surface.As indexing takes place in close conjunction with the drill bit 13 andnot through external rotation of the continuous drill support 4,problems such as “stick slip” can be avoided to a high degree.

In our own pending patent application no. SE 0104217-5 is shown anddescribed a new type of fluid-driven down-hole rock drill which herebyis incorporated as a reference in the present application. On thisdown-hole rock drill, the drill bit is mounted rotationally fixed butaxially limited in a bit sleeve, which in turn is supported in arotatable mounting in a housing included in the down-hole rock drill. Aram included in the drill machine is arranged to impact on the neck ofthe drill bit, a valve is arranged to control the reciprocating motionof the ram, wherein the valve alternately applies pressure and relievespressure in a chamber with a piston surface that when under pressuredrives the ram forward. One interesting feature of this drill machine,unlike conventional BHAs in which the respective units are arranged inline after each other in the direction of drilling, is that some of thedrive fluid used to activate the percussion ram is diverted to drive ameans of rotation arranged on the bit sleeve for indexing the bit sleeveand thereby the drill bit. This rotation means is so synchronised withthe ram that indexing is performed when the drill bit is in its rearmostend position or more precisely when it moves back away from the rocksurface.

This design contributes to the relatively short length of the BHA, whichoffers the advantage of the force that is applied via the flexible drillsupport being much closer to the working area of the drill bit. Itshould be understood that the controlling the drill bit with small curveradii is hereby facilitated and that only very little power is lost onthe way to the drill bit, even when the drill support diverts at a verysmall curve radius.

The present invention is not limited to the above description or asillustrated in the drawings but can be changed and modified in a numberof different ways within the framework of the idea of inventionspecified in the following claims.

1. Method of directional drilling of a hole, in particular when drillingcurved holes for mining purposes, drift mining or similar, for which isused a drill support in the form of a flexible continuous drill tubewith proximal end and distal end, that at the end of the drill support'sdistal end is connected a bottom hole assembly comprising a drillmachine and a percussion tool acting on a drill bit for forming a drillstring, which is fed into the gradually formed hole, characterised inthat the drill bit is indexed while drilling is in progress by graduallyrotating around its axis between each blow of the drill bit, theindexing is performed in the area between the drill support's distal endand the drill bit, the indexing taking place when the drill bit ismoving away from the rock surface or being in its retracted position,the indexing is carried out by means of a rotatable fluid activatedmotor the driving fluid for which is fed via a passage extended throughthe drill support.
 2. Method according to claim 1, whereby the drill bitrotates around the drill axis in defined steps by the fluid activatedmotor.
 3. Method according to claim 1, whereby the bottom hole assemblyis of the type that allows some of the drive flow used to activate thedrill bit action to be alternately diverted for rotating and indexingthe drill bit.
 4. Device for directional drilling of a hole, inparticular when drilling curved holes for mining purposes, drift miningor similar, comprising a drill support in the form of a flexiblecontinuous drill tube with a proximal end and a distal end and a bottomhole assembly comprising a drill machine and a percussion tool acting ona drill bit also allowing the drill bit to be rotated while drilling isin process the assembly being connected at the end of the drillsupport's distal part for forming a drill sting, and a means of feedingthe drill string into the gradually forming borehole, characterised inthat the bottom hole assembly comprises a fluid activated means forindexing and rotating the drill bit around an axis extending in thedirection of drilling while drilling is in progress, the indexing meansbeing so synchronized to the percussion tool that the indexing takingplace when the drill bit is moving away from the rock surface or beingin its retracted position, a passage extended through the drill supportfor supporting the indexing means with a driving fluid.
 5. Deviceaccording to claim 4, whereby the bottom hole assembly comprises a meansof control including a via a motor adjustable joint for controlling thedirection of the drill bit relative to the drill support.